There are many product streams in the field of petroleum chemistry which include both mono-olefins and di-olefins, for example resulting from hydrocracking, olefin oligomerization and paraffin dehydrogenation. For a variety of reasons it is desirable to separate mono-olefins from di-olefins. For example, normal alpha mono-olefins such as 1-butene, 1-hexene and 1-octene are valuable products; and product specifications often require relatively low levels of di-olefins (also known as dienes). When paraffins are dehydrogenated to form olefins and subsequently used in alkylation chemistry, di-olefin by-products from the dehydrogenation step produce undesired products in the alkylation step. Di-olefins are also known to poison some catalysts and need to be removed before certain reactions can be performed. Di-olefins themselves are often valuable products, but must be separated from unwanted mono-olefins. For example, di-olefins are often used as crosslinking agents and in Diels-Alder reactions.
Methods for selectively converting di-olefins to mono-olefins have been developed. For example, one commercially licensed process employs a selective, supported hydrogenation catalyst to selectively remove di-olefins by converting them to mono-olefins. However, there are several limitations to this type of process. Normal alpha olefins (NAO) may be isomerized to internal olefins in the presence of the catalyst. Hydrogenation can also cause side reactions such as formation of branched hydrocarbons. Some of the olefins may also be hydrogenated, forming paraffins. Further, the hydrogenation process does not remove paraffins, which are often present in the product streams. Additionally, known catalysts are toxic, corrosive, volatile and environmentally harmful.
Another method for separating di-olefins from mono-olefins involves distillation. However, di-olefins are difficult to remove from mono-olefins by distillation because they tend to be extremely close in relative volatility. Therefore, distillation requires a large number of stages and/or high reflux ratios. Paraffins are also difficult to separate from olefins via distillation because there is only a small difference in relative volatility between a paraffin and the corresponding olefin with the same number of carbons. Given purity requirements for commercially viable olefins, it is difficult and expensive to achieve the required separation using distillation.
Therefore, while the olefins are extremely commercially valuable, the commercially available methods for separating them are expensive, toxic, or both. None of the known processes provide a facile method for obtaining relatively high purity olefin components from olefin-containing streams such as cracked gases. It is a difficult separation to achieve economically by distillation. It is also difficult to control the side reactions and the migration of double bonds which occur in catalytic hydrogenation.
It would be advantageous to have economical methods for separating di-olefins from mono-olefins, and, preferably, separating both di-olefins and mono-olefins from non-olefins such as paraffins. It would also be advantageous to have such methods which also limit the formation of unwanted products, such as branched hydrocarbons, paraffins, and olefins with internal double bonds. The present invention provides such methods.
Ionic liquids are a category of compounds which are made up entirely of ions and are liquid at or below process temperatures. Usually, such compounds produce solids with high melting points (commonly known as ‘molten salts’). Ionic liquids differ from ‘molten salts’, in that they have low melting points, and are liquid at process temperatures. Moreover, they tend to be liquid over a very wide temperature range, with a liquid range of up to about 500° C. Ionic liquids are generally non-volatile, with no effective vapor pressure. Most are air and water stable, and are good solvents for a wide variety of inorganic, organic, and polymeric materials.
Ionic liquids are used herein to dissolve, suspend, disperse or otherwise immobilize olefin-complexing metal salts. When a mixture containing mono-and di-olefins is contacted with such an immobilized olefin-complexing metal salt, di-olefins are selectively complexed over mono-olefins, forming a metal salt/olefin complex. Since ionic liquids are non-volatile, the non-complexed mono-olefins may be easily separated via distillation or other conventional methods. Furthermore, the di-olefins may be readily desorbed and recovered from the metal salt/olefin complex, allowing the ionic liquid-metal salt solution to be recovered and recycled. Mono-olefins may also be complexed, allowing facile separation of non-olefins followed by selective desorption of mono- and di-olefins.